Лако-красочные материалы — производство

The use of structural adhesives requires not only essential choices between the many types available, but also a considered decision on the design approach appropriate to structures assembled with them. The simplest way forward currently could involve the following steps:

Fig. 4.22. Condition of the bonded interface of unstressed double lap joints after 2 years’ natural exposure. (Joints constructed with gritblasted steel adherends united by 5 types of epoxide adhesive, and cured at 20 °С).

(1) Adhesive selection and usage considerations from literature sources and/or proprietary software such as CADEPT (Harwell Laboratory), EASeL (Design Council), PAL (Permabond Adhesives), STICK (Lucas).

(2) Design with the aid of stress analysis programs (e. g. BISEPS, CADEPT, ESDU, PAL, PERA/CETIM, STICK), appropriate factors of safety and some reliability assessment method.

(3) Limited experimental work to investigate joint strength and durability.

Adhesives, as structural fasteners, are unique in that their stiffness can change with environmental conditions without any change in the loading on a structure which might be assembled with them. The adequate design of bonded joints must therefore take into account changes in the mechanical properties of adhesives, and the effect of such changes on the balance between the requirements of the different materials being joined. Rational design requires quantitative data on adhesives which requires, inter alia, appropriate test procedures for determining an envelope of their relevant structural properties. Probably the major deficiency in adhesive materials testing lies in the arbitrary test and report procedures adopted by the many investigators. Many test specimen configur­ations simply do not and cannot yield useful data, either to aid our understanding, or to be used for predicting performance in structural applications. In-bondline adhesive shear stress-strain response measurement is a useful method of characterising adhesives and of producing input data intended for structural analysis programs. However, bulk tensile response may yet prove to be a simple key element in design.

Proof of durability and safe performance are, rightly, onerous requirements for any innovations in the construction industry. The parameters affecting environmental durability have been summar­ised, and water has been identified as the most hostile environment for bonded joints that is commonly encountered. Identification of the general failure mechanisms is useful because it highlights the procedures necessary for the satisfactory fabrication of reliable and durable bonded joints. It also enables the development and adoption of appropriate test methods, since ‘real’ joint configurations are of limited use in assessing experimentally environmental effects (e. g. bonded areas must be minimised in order to allow environmental access within a reasonable time-scale). Fracture mechanics methods, especially those employing self-stressed cleavage joints, can provide an excellent means of examining adhesion rapidly, estimating bond durability, and for yielding data for direct use in design. Long-term joint durability is greatly dependent upon the stability of the adhesive-adherend interface. It follows that surface pretreatment is critical, and this is likely to be the most difficult aspect to control, particularly on site.

Real applications demand quality assurance in the form of appropriate physical and mechanical test procedures carried out alongside the actual fabrication. Naturally they should reflect the nature of the processes involved rather than, as in the ubiquitous concrete cube test, something almost totally unrelated. The value of a reliable method for assessing the post-bonding quality of bonded joints in the context of civil engineering applications will be self — evident. The application of non-destructive tests to bonded joints in general has been reviewed elsewhere(5). An interim solution which would at least provide the opportunity for strength tests would be to design joints from which test coupons could be removed, perhaps with a view to the long-term as well as to the short.